This invention relates to a method for continuous saccharification of cellulose of plant raw material by feeding raw material and/or the same material prehydrolyzed and a dilute sulfuric acid solution into a flow reactor in order to hydrolyze the raw material in pressurized atmosphere at high temperature, by discharging solid and liquid from the reactor by expansion and by separating the liquid containing sugars from the solid.
The present invention thus relates to a method for decomposing the hemicellulose and cellulose contained in various plant materials by hydrolysis into monosaccharides with diluted acid. The monosaccharides are useful raw materials both for the chemical and microbiologic industries. While the prices of petrochemical products are continuously rising the prices of products which are based on plant raw material such as for example ethanol and its derivates and protein are gradually becoming competitive and interest taken in these products is continuously growing. The object of the present invention is to accomplish a method for producing monosaccharides from cellulose-containing plant materials which can be used as raw materials for chemical and microbiologic industries.
All materials containing cellulose or lignocellulose, such as paper waste, straw, bagasse, saw dust, wood chips and peat are suitable for the method according to the invention.
Several processes are known to hydrolyze plant raw materials containing cellulose with a dilute water solution of sulfuric acid. These known processes are mainly based on the so-called Scholler process which was one of the first hydrolysis processes industrially applied. In the Scholler process plant raw material is hydrolyzed in batches in a percolator. In the first treatment a dilute sulfuric acid solution is led through the plant raw material to be hydrolyzed at a temperature of 150.degree.-160.degree. C., and in the second treatment a little stronger sulfuric acid at 180.degree.-200.degree. C. is led through the treated plant raw material as quickly as possible in order to prevent decomposition of the hydrolyzed sugars.
A disadvantage of the Scholler process is the very long duration of the treatment which takes many hours and demands thus several expensive and bulky percolators, besides which the sugar content of the hydrolyzate and the sugar yield remain low. In addition to this it has proven to be difficult to make liquid pass evenly through the plant raw material to be hydrolyzed because during the progress of the hydrolysis the plant material becomes finer and channels are built in it through which the liquid passes while the material between the channels remains substantially unhydrolyzed.
Finnish Pat. No. 51370 discloses a method for continuous saccharification of cellulose of solid plant raw material wherein the plant raw material is continuously hydrolyzed in one reactor in two stages. The continuous flow reactor for the main hydrolysis is set below the prehydrolysis reactor and is an immediate continuation thereof. In the reactor the liquid flows faster than the solid particles, in other words the liquid flows through the plant raw material to be hydrolyzed in accordance with percolation principle. The disadvantages associated with the Scholler process are not eliminated by this process either. Also in this case channels are formed in the solid material. Liquid flows through these channels while the material between the channels remains substantially unhydrolyzed.
In the method according to this patent residue and liquid are discharged from the reactor by means of expansion; by blowing the liquid and the residue separately through the reactor bottom into blow tanks. Similarly to the Scholler process, also here relatively great amounts of water are used, i.e. 9 to 3 kilos of liquid to one kilo of dry solids contained in the raw material. When the residue is blown from the reactor separately, it is possible to evaporate liquid from the residue.
The plant raw material, however, contains different kinds of particles, some of which are hydrolyzed quicker than the others. In percolation type processes this is taken into consideration by letting liquid flow through the reactor faster than solid. Thus the more easily hydrolyzed particles can be discharged from the reactor earlier than the less easily hydrolyzable ones, and so the yield of sugar is increased. However, it has turned out that when liquid and solid are flowing at different speeds in the reactor, channels are formed in the solid through which the liquid mainly passes. Therefore a great part of the solid does not react and contains still unhydrolyzed particles when leaving the reactor.
The object of the present invention is to eliminate the above-mentioned disadvantages and to provide a method for continuous separation of sugar from plant raw material with high sugar yield, high sugar content and low energy consumption as well as at the lowest possible investment costs.